The present invention relates to an image processing method and an image forming device and particularly to an image processing method that may be appropriately used to correct inconsistent density arising from a variation in characteristics among recording elements of a recording head and an image forming device using this image processing method.
Note that the inconsistent density as used herein includes inconsistency attributable to a nozzle's failure to discharge ink, as will be described.
An image forming device (e.g., ink jet printer) provided with an ink jet type recording head having ink discharge nozzles is liable to develop an inconsistent density (stripes) in a recorded image because of a variation in ink discharge characteristics among nozzles (e.g., discharge direction, discharge amount, ink drop amount, and failure to discharge). The nozzle's discharge characteristics, a main cause of stripes, may be broken down to landing position error in a direction in which the nozzles are arranged, drop amount error, failure to discharge, and the like. These nozzle's discharge characteristics cause inconsistent density in the form of stripes.
While, as is known in the art, inconsistent density can be prevented by a multi-pass printing in the case of a shuttle scan type image recording device where image recording is accomplished by causing the recording head to scan a given printing area a plurality of times, preventing inconsistent density as described above is difficult with the line-head type that accomplishes image recording in one scan.
However, most of the image forming devices (e.g., ink jet printers) intended to offer a high speed and a high accuracy perform a single-pass drawing using line heads as described above. In such a case, multi-nozzle recording heads having an output resolution as high as say about 1200 dpi are used to achieve a high-quality image. To achieve such a resolution, marked ink dots each having a diameter of 30 μm or greater may be used in some applications to fill up each space (p×√{square root over (2)}=about 30 μm, where pitch p=21.2 μm) of a grid having a resolution of 1200 dpi×1200 dpi.
With a printer such as one of shuttle scan type described above that permits change of resolutions according to the scan mode, a plurality of resolutions meeting various purposes intended are set and dots with matching diameters are provided so as to achieve an optimum image quality and productivity in most of the cases. With a single-pass printer as mentioned above, the resolution is fixed, and a single dot diameter is provided to meet normal output conditions.
As the number of nozzles increases, a single-pass printer as mentioned above is liable, as expected, to develop flaws in nozzles with a certain probability. A flaw in recording characteristics of a nozzle causes an image defect (inconsistent density in stripes), and various methods have been proposed to address this problem of inconsistent density.
Presently, various inconsistent density correction methods are used. By these methods, inconsistent density is corrected basically by changing the density in the output image according to the characteristics of the respective recording elements. The methods may be broken down to two types: one whereby discharge drive conditions specific to each recording element is set to adjust dot diameters and dot densities, and the other whereby image data or dot densities (number of dots) are varied to correct the inconsistent density.
Out of the two methods, the latter is used more widely because the former method is limited in the type of heads that may be used and a range by which correction can be made, while the latter permits a greater freedom.
For example, JP 2006-264069 A discloses a technique for measuring the densities of areas corresponding to the respective recording element positions to correct the inconsistent density of the corresponding printing area. JP 2007-160748 A discloses a method for efficiently and accurately calculating a density correction coefficient from a characteristics error of recording elements (marked ink dot interval error).
To convert image data by the inconsistent density correction method, a 1D-LUT that is specific to each recording element is used to effect γ conversion. There are two methods of obtaining a correction curve (inconsistency correction coefficient) of the 1D-LUT: one whereby, as described in JP 2006-264069 A, the densities of areas corresponding to the respective recording element positions are measured to correct the inconsistent density of the corresponding printing area and the other whereby, as described in JP 2007-160748 A, a drop discharge position accuracy of a recording element is measured accurately to obtain a correction coefficient from the position information.
In recent years, image forming devices intended to offer a high speed and a high accuracy use line heads to perform a single-pass drawing in most of the cases. Accordingly, where a multi-nozzle recording head having an output resolution of, for example, 1200 dpi is used, a marked ink dot interval error must be held to a minimum.
The technique disclosed in JP 2007-160748 A is capable of accurately correcting an inconsistency if a marked dot position can be measured as recording element information. Where the marked dot position accuracy is poor (position error is great), however, this technique can develop a flaw when a calculated correction coefficient is applied to a particular image.
For example, FIGS. 10A and 10B illustrate a case where a nozzle nzl4 draws a line by successively discharging 6 marked ink dots. As illustrated in FIG. 10A, when a nozzle nzl3 has an error ΔX=0, a dot is marked at the position of the nozzle nzl4; as illustrated in FIG. 10B, when ΔX=0.4 L, for example, the density decreases by 30% and when Δx=0.7 L, the line totally disappears.
The value ΔX represents an error from an ideal dot position by a ratio to an ideal distance L. For example, when ΔX=1.0 L, the two dots overlap entirely.
For reference, FIG. 11 is a block diagram illustrating an inconsistency correction technique practiced in the art; FIG. 12 is a flow chart of operations corresponding to FIG. 11.